The present invention relates to benzoxazines. In a particular aspect, the present invention relates to benzoxazine-containing thermosetting resin compositions. In yet another aspect, the present invention relates to compositions useful as adhesives for assembling electronic devices, as well as other applications, such as within the aerospace industry.
Reliable performance of electronic devices depends primarily on the integrity of the microelectronic components contained therein. Most electronic devices contain several microchips which are housed in a variety of protective packages. These packages are composed of several distinct materials, each of which performs a specific function and contributes to the overall integrity of the package. For example, a typical ball-grid array (BGA) package contains, in addition to the substrate and microchip attached thereto, materials such as overmold adhesive, die-attach adhesive, and solder mask. The incorporation of all of these disparate materials into one package creates several adhesive interfaces within the package itself. In order to produce a reliable, long-lasting electronic device, the structural integrity of these interfaces must be maintained.
Interfacial adhesion is a critical parameter for the production of reliable microelectronic components. Materials with dissimilar coefficients of thermal expansion must be adhered via void-free bonds. The presence of any delamination in the final assembled product can lead to moisture entrapment within the void volume. The trapped moisture can be released xe2x80x9cexplosivelyxe2x80x9d once the defective part is heated to solder reflow temperatures during final component assembly. The formation of sound adhesive bonds at the various interfaces present within a microelectronic package is therefore critical for the survival of that package during assembly. The formation of sound adhesive bonds is also necessary to insure a long service life for the final product.
Adhesive interfaces within electronic components are currently subjected to increasingly stringent processing conditions. For example, environmental concerns have resulted in a worldwide mandate to remove lead from all aspects of the microelectronic assembly process. The use of lead-free solder alloys, however, creates a new challenge for the reliable assembly of microelectronic components. The reflow temperatures required by lead-free alloys are several degrees higher than those containing lead. Soldering operations based on these new alloys generally must be conducted around 260xc2x0 C., which is about forty degrees Celsius higher than had been previously required. The new, higher reflow temperatures place an extra strain on all of the adhesive interfaces within microelectronic packages. Indeed, strong adhesion at all of these interfaces is critical to the reliability of a microelectronic component.
Benzoxazines and compositions containing benzoxazines are known (see for example, U.S. Pat. Nos. 5,543,516 and 6,207,786 to Ishida, et. al.; S. Rimdusit and H. Ishida, xe2x80x9cDevelopment of New Class of Electronic Packaging Materials Based on Ternary Systems of Benzoxazine, Epoxy, and Phenolic Resinsxe2x80x9d, Polymer, 41, 7941-49 (2000); and H. Kimura, et. al., xe2x80x9cNew Thermosetting Resin from Bisphenol A-based Benzoxazine and Bisoxazolinexe2x80x9d, J. App. Polym. Sci., 72, 1551-58 (1999)). However, benzoxazines have generally not been used as components of thermosetting resin compositions to increase the interfacial adhesion thereof.
Accordingly, there is a need for compositions and methods which increase interfacial adhesion within microelectronic components.
In accordance with the present invention, there are provided novel benzoxazine compounds and thermosetting resin compositions prepared therefrom. Invention compositions are particularly useful for increasing adhesion at interfaces within microelectronic packages. Invention benzoxazines are useful for the preparation of invention compositions with properties which are associated with increased adhesion at interfaces, such as, for example, low shrinkage on cure and low coefficient of thermal expansion (CTE).
In another aspect of the invention, there are provided die-attach pastes having increased interfacial adhesion. Invention die-attach pastes include benzoxazine-containing thermosetting resin compositions.
In further aspects of the invention, there are provided methods for enhancing adhesive strength of thermosetting resin compositions and methods for enhancing adhesion of a substrate bound to a metallic surface by a thermosetting resin composition.
In accordance with the present invention, there are provided benzoxazines having the following structure: 
wherein:
L is an optional alkylene or siloxane linking moiety,
Ar is optionally substituted arylene,
Q is an oxazine ring or amine salt thereof having the structure: 
and is bonded to Ar in a fused manner at positions 5 and 6 of the oxazine ring, wherein:
Sp is optional, and if present, is an optionally substituted C1 to C6 alkylene, oxyalkylene, thioalkylene, carboxyalkylene, amidoalkylene, or sulfonatoalkylene spacer,
n is 1 or 2,
m is optional, and if present, is 1 or 2,
x and y are each independently 0 to 4, and
wherein at least one of R, Rxe2x80x2, and Rxe2x80x3 is a polymerizable moiety.
As employed herein, xe2x80x9carylenexe2x80x9d refers to aromatic groups having in the range of 6 up to 14 carbon atoms and xe2x80x9csubstituted arylenexe2x80x9d refers to arylene groups further bearing one or more substituents selected from hydroxy, alkyl, alkoxy, mercapto, cycloalkyl, substituted cycloalkyl, heterocyclic, substituted heterocyclic, aryl, substituted aryl, heteroaryl, substituted heteroaryl, aryloxy, substituted aryloxy, halogen, cyano, nitro, nitrone, amino, amido, C(O)H, acyl, oxyacyl, carboxyl, carbamate, sulfonyl, sulfonamide, sulfuryl, and the like.
As employed herein, xe2x80x9calkylenexe2x80x9d refers to divalent hydrocarbyl radicals having 1 up to 20 carbon atoms, preferably 2-10 carbon atoms, and xe2x80x9csubstituted alkylenexe2x80x9d refers to alkylene moieties bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9coxyalkylenexe2x80x9d refers to an alkylene moiety wherein one or more of the carbon atoms have been replaced by oxygen atoms, and xe2x80x9csubstituted oxyalkylenexe2x80x9d refers to an oxyalkylene moiety further bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9cthioalkylenexe2x80x9d refers to an alkylene moiety wherein one or more of the carbon atoms have been replaced by sulfur atoms, and xe2x80x9csubstituted thioalkylenexe2x80x9d refers to an thioalkylene moiety further bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9ccarboxyalkylenexe2x80x9d refers to an alkylene moiety wherein one or more of the carbon atoms have been replaced by a carboxyl group, and xe2x80x9csubstituted carboxyalkylenexe2x80x9d refers to a carboxyalkylene moiety further bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9camidoalkylenexe2x80x9d refers to an alkylene moiety wherein one or more of the carbon atoms have been replaced by an amido group, and xe2x80x9csubstituted amidoalkylenexe2x80x9d refers to an amidoalkylene moiety further bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9csulfonatoalkylenexe2x80x9d refers to an alkylene moiety wherein one or more of the carbon atoms have been replaced by a sulfonato group, and xe2x80x9csubstituted sulfonatoalkylenexe2x80x9d refers to a sulfonatoalkylene moiety further bearing one or more of the substituents as set forth above.
As employed herein, xe2x80x9cpolymerizable moietyxe2x80x9d refers to any substituent that can participate in polymerization reaction, such as, for example, an addition polymerization or a condensation polymerization. As employed herein, addition polymerization refers to polymerization mechanisms such as free-radical polymerization, anionic polymerization, cationic polymerization, ring-opening polymerization, or coordinative polymerization. As employed herein, condensation polymerization refers to polymerizations such as siloxane polymerization.
In one aspect of the invention, the polymerizable moiety participates in an addition polymerization. Preferred addition polymerizable moieties include, for example, optionally substituted alkenyl, oxyalkenyl, alkynyl, cycloalkenyl, bicycloalkenyl, styryl, (meth)acrylate, itaconate, maleimide, vinyl ester, epoxy, cyanate ester, nitrile, diallyl amide, benzocyclobutene, aromatic propargyl ether, aromatic acetylene, oxazoline, and the like. Most preferred addition polymerizable moieties include alkenyl, oxyalkenyl, (meth)acrylate, maleimide, or cycloalkenyl.
As employed herein, xe2x80x9calkylxe2x80x9d refers to hydrocarbyl radicals having 1 up to 20 carbon atoms, preferably 2-10 carbon atoms; and xe2x80x9csubstituted alkylxe2x80x9d comprises alkyl groups further bearing one or more substituents selected from hydroxy, alkoxy (of a lower alkyl group), mercapto (of a lower alkyl group), cycloalkyl, substituted cycloalkyl, heterocyclic, substituted heterocyclic, aryl, substituted aryl, heteroaryl, substituted heteroaryl, aryloxy, substituted aryloxy, halogen, trifluoromethyl, cyano, nitro, nitrone, amino, amido, C(O)H, acyl, oxyacyl, carboxyl, carbamate, sulfonyl, sulfonamide, sulfuryl, and the like.
As employed herein, xe2x80x9calkenylxe2x80x9d refers to straight or branched chain hydrocarbyl groups having at least one carbonxe2x80x94carbon double bond, and having in the range of about 2 up to about 12 carbon atoms, and xe2x80x9csubstituted alkenylxe2x80x9d refers to alkenyl groups further bearing one or more substituents
As employed herein, xe2x80x9calkoxyxe2x80x9d refers to the moiety xe2x80x94O-alkyl-, wherein alkyl is as defined above, and xe2x80x9csubstituted alkoxyxe2x80x9d refers to alkoxy groups further bearing one or more substituents as set forth above.
As employed herein, xe2x80x9calkynylxe2x80x9d refers to straight or branched chain hydrocarbyl groups having at least one carbonxe2x80x94carbon triple bond, and having in the range of about 2 up to about 12 carbon atoms, and xe2x80x9csubstituted alkynylxe2x80x9d refers to alkynylene groups further bearing one or more substituents as set forth above.
As employed herein, xe2x80x9ccycloalkylxe2x80x9d refers to cyclic ring-containing groups containing in the range of about 3 up to about 8 carbon atoms, and xe2x80x9csubstituted cycloalkylxe2x80x9d refers to cycloalkyl groups further bearing one or more substituents as set forth above.
As employed herein, xe2x80x9ccycloalkenylxe2x80x9d refers to cyclic ring-containing groups containing in the range of about 3 up to about 8 carbon atoms and having at least one carbonxe2x80x94carbon double bond, and xe2x80x9csubstituted cycloalkenylxe2x80x9d refers to cycloalkenyl groups further bearing one or more substituents as set forth above.
As employed herein, xe2x80x9carylxe2x80x9d refers to aromatic groups having in the range of 6 up to about 14 carbon atoms and xe2x80x9csubstituted arylxe2x80x9d refers to aryl groups further bearing one or more substituents as set forth above.
As employed herein, xe2x80x9cheteroarylxe2x80x9d refers to aromatic groups containing one or more heteroatoms (e.g., N, O, S, or the like) as part of the ring structure, and having in the range of 6 up to about 14 carbon atoms.
As employed herein, xe2x80x9cheterocyclicxe2x80x9d refers to cyclic (i.e., ring-containing) groups containing one or more heteroatoms (e.g., N, O, S, or the like) as part of the ring structure, and having in the range of 3 up to 14 carbon atoms and xe2x80x9csubstituted heterocyclicxe2x80x9d refers to heterocyclic groups further bearing one or more substituents as set forth above.
In another aspect of the invention, the polymerizable moiety participates in a condensation polymerization. Preferred condensation polymerizable moieties include, for example, siloxanes
Invention benzoxazine monomers are preferably liquids and may optionally contain additional functionality depending on the specific application, such as, for example, alcohols, amines, silane esters, thiols, isocyanates, anhydrides, and the like.
In one embodiment, invention benzoxazines have the following structure:
Rxxe2x80x94Ar(Q)n 
wherein:
Ar is optionally substituted arylene,
Q is an oxazine ring or amine salt thereof having the structure: 
and is bonded to Ar in a fused manner at positions 5 and 6 of the oxazine ring,
Sp is optional, and if present, is a C1 to C6 alkylene oxyalkylene, thioalkylene, carboxyalkylene, amidoalkylene, or sulfonatoalkylene spacer,
n is 1 or 2,
x and y are each independently 1 to 4, and
wherein at least one of R, Rxe2x80x2, or Rxe2x80x3 is a polymerizable moiety.
Invention benoxaziness may contain one or two oxazine rings per aryl ring, represented by the following exemplary structures A and B, respectively: 
Presently preferred benzoxazines having the above exemplary structures include the following: 
In another aspect, said optionally substituted arylene is naphthylene, represented by the following exemplary structure C: 
Presently preferred benzoxazines having exemplary structure C include the following: 
In another aspect of the invention, benzoxazines contain an optional linker moiety L. A variety of groups are contemplated for use as a linker, such as, for example, alkylene or siloxane groups. The linking moiety can link benzoxazine groups in a variety of ways, e.g., via the aromatic rings or via the nitrogen atoms of the oxazine rings, as shown in the following exemplary structures D and E, respectively: 
Benzoxazines containing optional linking groups L have the following exemplary structures: 
Invention benzoxazines are readily prepared in one step from an aromatic alcohol, formaldehyde, and primary amine, as shown in Scheme 1: 
Functionality can be readily incorporated into invention benzoxazines by using a substituted aromatic alcohol and/or substituted primary amine in the synthesis outlined in Scheme 1.
In another aspect of the invention, there are provided thermosetting resin compositions containing one or more of the above described benzoxazines. Inventions compositions exhibit markedly increased adhesion to a variety of substrates, such as, for example, copper, aluminum, silicon, and the like. Thus, invention compositions are particularly useful for assembling microelectronic components.
Thermoset chemistries contemplated for use in the practice of the present invention include epoxy, cyanate ester, maleimide, acrylate, methacrylate, vinyl ether, styrenic, vinyl ester, propargyl ether, diallylamide, aromatic acetylene, benzocyclobutene, thiolene, maleate, oxazoline, itaconate, and the like, as well as combinations of any two or more thereof.
A presently preferred thermoset material contemplated for use in the practice of the present invention is maleimide. Maleimides contemplated for use in the practice of the present invention have the following structure: 
wherein:
m is 1-3,
each R is independently hydrogen or lower alkyl, and
X is a saturated straight chain or branched chain alkyl, alkylene, or alkylene oxide, optionally containing saturated cyclic moieties as substituents on said alkyl, alkylene or alkylene oxide chain or as part of the backbone of the alkyl, alkylene or alkylene oxide chain.
Invention compositions exhibit significantly increased adhesive strength after incorporation of only small amounts of invention benzoxazines. Thus, invention benzoxazines are generally incorporated at levels in the range of about 0.5 up to about 25 wt % of the total thermoset monomers present in the composition. Presently preferred levels of incorporation are in the range of about 1 up to about 10 wt % of the total thermoset monomers present. Presently most preferred levels of incorporation are in the range of about 1 up to about 5 wt % of the total thermoset monomers present.
When invention benzoxazines are contemplated for use as components of free-radically cured thermosets, it is important to keep in mind that all benzoxazines contain a tertiary amine group. It is known that tertiary amines may inhibit free-radical curing reactions. Thus, to minimize the impact of tertiary amine groups on the free-radical cure, the tertiary amines can be converted to an amine salt by the addition of one equivalent of a suitable acid (see Scheme 2). 
It is especially desirable to use an acid that is itself capable of participating in a free radical cure (i.e., Rxe2x80x2 in Scheme 2 is a moiety capable of polymerizing free-radically). Examples of acidic, free-radically-curable compounds that can be used to form amine salts include monobasic acids such as acrylic, methacrylic, 2-carboxyethyl acrylate, maleimidopropionic, maleimidocaproic, cinnamic, 4-styrenesulfonic, 4-vinylbenzoic, undecylenic, vinylsulfonic acids, and the like. Examples of dibasic acids that can be used for this purpose include maleic, furmaric, itaconic, vinylphosphonic acids, and the like.
An alternative method that may be used to reduce the free-radical cure inhibition of the tertiary amine is to oxidize the amine to an N-oxide (see Scheme 3). 
Conversion of the tertiary amine residues to N-oxides may be accomplished through the use of oxidizing agents such as peracetic acid, hydrogen peroxide, and the like. Procedures to perform this oxidation are well known to those familiar with the art.
In addition, it is of note that benzoxazines contemplated for use in the practice of the present invention need not necessarily crosslink with other thermoset materials in the composition during cure to provide beneficial properties. For example, incorporation of benzoxazine compounds that contain no free-radically polymerizable group into a peroxide catalyzed, maleimide-based thermosetting composition remarkably increases the tensile, room temperature bond strength to copper and aluminum. Use of invention benzoxazines is also shown herein to be useful as a means to retain (or in some cases improve) the adhesion of compositions subjected to extended high temperature storage.
Optionally, invention compositions can further contain one or more of the following additional components: anti-oxidants, bleed control agents, one or more fillers, inert (i.e., nonreactive) diluents, reactive diluents, coupling agents, adhesion promoters, flexibilizers, dyes, pigments, and the like.
Anti-oxidants contemplated for optional use in the practice of the present invention include hindered phenols (e.g., BHT (butylated hydroxytoluene), BHA (butylated hydroxyanisole), TBHQ (tertiary-butyl hydroquinone), 2,2xe2x80x2-methylenebis(6-tertiarybutyl-p-cresol), and the like), hindered amines (e.g., diphenylamine, N,Nxe2x80x2-bis(1,4-dimethylpentyl-p-phenylene diamine, N-(4-anilinophenyl) methacrylamide, 4,4xe2x80x2-bis(xcex1,xcex1-dimethylbenzyl) diphenylamine, and the like), phosphites, and the like. When used, the quantity of anti-oxidant typically falls in the range of about 100 up to about 2000 ppm, relative to the weight of the base formulation.
Bleed control agents contemplated for optional use in the practice of the present invention include cationic surfactants, tertiary amines, tertiary phosphines, amphoteric surfactants, polyfunctional compounds, and the like, as well as mixtures of any two or more thereof. Those of skill in the art recognize that the quantity of bleed control agent employed in the practice of the present invention can vary widely, typically falling in the range of about 0.1 up to about 10 wt %, relative to the weight of the base formulation.
Fillers contemplated for optional use in the practice of the present invention may optionally be conductive (electrically and/or thermally). Electrically conductive fillers contemplated for use in the practice of the present invention include, for example, silver, nickel, gold, cobalt, copper, aluminum, graphite, silver-coated graphite, nickel-coated graphite fillers, alloys of such metals, and mixtures thereof, and the like. Both powder and flake forms of filler may be used in the die-attach paste compositions of the present invention. Preferably, the flake has a thickness of less than about 2 microns, with planar dimensions of about 20 to about 25 microns. Flake contemplated for use herein preferably has a surface area of about 0.15 to 5.0 m2/g and a tap density of about 0.4 up to about 5.5 g/cc. It is presently preferred that powder employed in the practice of the invention has a diameter in the range of about 0.5 up to about 15 microns.
Thermally conductive fillers contemplated for optional use in the practice of the present invention include, for example, aluminum nitride, boron nitride, silicon carbide, diamond, graphite, beryllium oxide, magnesia, silica, alumina, and the like. Preferably, the particle size of these fillers will be about 20 microns. If aluminum nitride is used as a filler, it is preferred that it be passivated via an adherent, conformal coating (e.g., silica, or the like).
Electrically and/or thermally conductive fillers are optionally (and preferably) rendered substantially free of catalytically active metal ions by treatment with chelating agents, reducing agents, nonionic lubricating agents, or mixtures of such agents. Such treatment is described in U.S. Pat. No. 5,447,988, which is incorporated by reference herein in its entirety.
Optionally, a filler may be used that is neither an electrical nor thermal conductor. Such fillers may be desirable to impart some other property such as reduced dielectric constant, improved toughness, increased hydrophobicity, and the like. Examples of such fillers include perfluorinated hydrocarbon polymers (i.e., TEFLON(trademark)), thermoplastic polymers, thermoplastic elastomers, mica, fused silica, and the like.
While the use of inert diluents is not excluded from the practice of the present invention, it is generally preferred that compositions according to the invention remain substantially free of solvent, so as to avoid the potentially detrimental effects thereof, e.g., creation of voids caused by solvent escape, the environmental impact of vaporized solvent, the redeposition of outgassed molecules on the surface of the article, and the like. When used, suitable inert diluents include dimethylformamide, dimethylacetamide, N-methylpyrrolidone, toluene, xylene, methylene chloride, tetrahydrofuran, glycol ethers, methyl ethyl ketone or monoalkyl or dialkyl ethers of ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, and the like. When used, inert diluents are typically present in the range of about 10 up to about 40 wt %, relative to the weight of the base formulation.
Reactive diluents contemplated for optional use in the practice of the present invention include any reactive diluent which, in combination with the maleimide-based benzoxazine-containing formulations described herein, forms a thermosetting resin composition. Such reactive diluents include acrylates and methacrylates of monofunctional and polyfunctional alcohols, ethylenically unsaturated compounds, styrenic monomers (i.e., ethers derived from the reaction of vinyl benzyl chlorides with mono-, di-, or trifunctional hydroxy compounds), and the like. When used, reactive diluents are typically present in the range of about 5 up to about 15 wt %, relative to the weight of the base formulation.
In a particular aspect, compositions according to the invention optionally further contain in the range of about 0.1 up to about 10 wt % of at least one coupling agent, based on the total weight of the composition. Coupling agents contemplated for use in the practice of the present invention include silicate esters, metal acrylate salts, titanates or compounds containing a co-polymerizable group and a chelating ligand.
As employed herein, the term xe2x80x9cadhesion promotersxe2x80x9d refers to components (other than invention compounds) which have pendant acid or latent acid groups that can increase adhesion. An example is the Ricon R-130 20% maleated (Ricon Resins, Inc., Grand Junction, Colo.), a polybutadiene with anhydride groups that can react with a surface to increase adhesion. When present, adhesion promoters are typically present in the range of about 5 up to aboiut 30 wt %, relative to the weight of the base formulation.
Flexibilizers contemplated for optional use in the practice of the present invention include branched polyalkanes or polysiloxanes that lower the Tg of the formulation. An example of such a material would be polybutadienes such as the Ricon R-130 as described hereinabove. When present, flexibilizers are typically present in the range of about 15 up to about 60 wt %, relative to the weight of the base formulation.
Dyes contemplated for optional use in the practice of the present invention include nigrosine, Orasol blue GN, phthalocyanines, and the like. When used, organic dyes in relatively low amounts (i.e., amounts less than about 0.2 wt %) provide contrast.
Pigments contemplated for optional use in the practice of the present invention include any particulate material added solely for the purpose of imparting color to the formulation, e.g., carbon black, metal oxides (e.g., Fe2O3, titanium oxide), and the like. When present, pigments are typically present in the range of about 0.5 up to about 5 wt %, relative to the weight of the base formulation.
Those of skill in the art recognize that many different electronic packages would benefit from preparation using the invention compositions described herein. Examples of such packages include ball grid arrays, super ball grid arrays, IC memory cards, chip carriers, hybrid circuits, chip-on-board, multi-chip modules, pin grid arrays, chip size packages (CSPs), and the like. Thus, in a further aspect of the invention, there are provided die-attach pastes comprising invention compositions and optionally, a filler. Preferably, invention die-attach pastes comprise in the range of about 10 up to about 80 wt % of invention composition and in the range of about 20 up to about 90 wt % of a filler.
Invention compositions and die-attach pastes are particularly useful for enhancing the adhesion of a substrate to a metallic surface. In a particular aspect, the substrate is a semiconductor die, and the metallic surface is copper.
In still further aspects of the invention, there are provided methods for enhancing adhesive strength of a thermosetting resin composition and enhancing adhesion of a substrate bound to a metallic surface by a thermosetting resin composition, wherein the methods comprise incorporating an effective amount of one or more invention compounds into the composition. In a preferred embodiment of this aspect of the invention, the metallic surface is copper.
In yet another aspect of the invention, there is provided a method for adhesively attaching a substrate to a metallic surface, wherein the method comprises curing a die-attach paste positioned between the substrate and the metallic surface, wherein the die-attach paste comprises one or more invention compounds. In a preferred embodiment of this aspect of the invention, the substrate is a semiconductor die and the metallic surface is a lead frame. In an especially preferred embodiment, the lead frame is a copper lead frame.
The invention will now be described in greater detail by reference to the following non-limiting examples.
For each of the following Examples, the formation of the benzoxazine was confirmed by IR spectroscopy, wherein the disappearance of the characteristic xe2x80x94OH stretch of the phenol starting material was accompanied by the observance of the characteristic Rxe2x80x94Oxe2x80x94R stretch of the oxazine ring at 1080 cmxe2x88x921.